This invention generally relates to a cooling system for a high-speed propulsion system, and specifically to a cooling system including a fuel deoxygenator and a catalyst for increasing the heat sink capability of a hydrocarbon fuel capable of undergoing endothermic reaction.
It is common practice to use fuel as a cooling medium for various systems onboard an aircraft. Higher engine operating temperatures increase cycle efficiency and reduce fuel consumption. The engine operating temperature is limited by the usable cooling capacity of the fuel. The cooling capacity of fuel can be increased by endothermic decomposition of the fuel into combustible products that may have improved ignition and burning characteristics.
Catalysts are known that promote decomposition of endothermic fuels into combustible products with lower molecular weights than the original fuel after absorbing a heat of reaction. However, thermo-oxidative reactions caused by dissolved oxygen within the fuel can cause formation of coke that foul the catalyst and prevent the preferred catalytic reactions.
At temperatures between approximately 250° F. to 800° F. dissolved oxygen within the fuel reacts to form coke precursors that initiate and propagate reactions that lead to coke deposit formation. At temperatures above approximately 800° F. the mechanism for formation of coke deposits is controlled by thermal cracking (pyrolysis) reactions where chemical bonds are broken forming coke. Reducing the amount of oxygen dissolved within the fuel decreases the rate of coke deposition at relatively lower temperatures and increases the usable cooling capacity of the fuel.
It is known how to remove dissolved oxygen within fuel with de-oxygenation devices. U.S. Pat. No. 6,315,815, and U.S. patent application Ser. No. 10/407,004 assigned to Applicant, disclose devices for removing dissolved oxygen using a gas-permeable membrane within the fuel system. As fuel passes along the permeable membrane, oxygen molecules in the fuel diffuse out of the fuel across the gas-permeable membrane. Removal of dissolved oxygen from the fuel only provides limited increases in usable cooling capacity. Increasing performance demands require further increases in the usable cooling capacity of aircraft fuels.
Accordingly, it is desirable to develop an endothermic fuel system that suppresses formation of coke deposits to prevent interference with desirable catalytic reactions at increased engine operating temperatures.